The present invention relates to device-specific information for pixels.
Color in computer graphics is defined in terms of “color spaces”, which are related to real or imaginary color display devices such as monitors, liquid crystal displays and color printers. Various color spaces are used to represent color on computers. Each image is associated with a color space which defines colors according to a combination of properties. For example, in a RGB (Red, Green, Blue) color space, each color is represented by a combination of red, green and blue components. In a CMYK (Cyan, Magenta, Yellow, Black) color space, each color is represented as a combination of cyan, magenta, yellow and black.
An output display device such as a computer monitor, liquid crystal display (LCD) or printer is capable of reproducing a limited range of colors. An output display device's “color gamut” is the set of colors that the output display device is capable of reproducing. Similarly, the “visible color gamut” is the set of colors that the human eye is capable of perceiving. Color gamuts can be represented as a two-dimensional projection of their three-dimensional representations onto the plane of constant luminance.
Typically, color display devices are constructed from an array of pixels that are themselves composed of several (typically three) differently colored components or sub-pixels. The input values of these sub-pixels may be varied from full off to full on to cause the output display device to display the pixels at visual output intensities corresponding to the pixel input values. The perceived color of each pixel is the aggregate of the visual output intensities and colors of its sub-pixels. Thus, a pixel can take a range of values through the color spectrum by varying the input values of its sub-pixels.
A pixel's color is generally represented by a series of bits (the “color value”), with specific bits indicating a visual output intensity for each sub-pixel used in the color. The specific sub-pixels depend on the color system used. Thus, a 24-bit RGB data representation may allocate bits 0–7 to indicate the amount of blue, bits 8–15 to indicate the amount of green, and bits 16–23 to indicate the amount of red, as shown in FIG. 3. Such a representation can produce any one of nearly 17 million different pixel colors (i.e., the number of unique combinations of 256 input values of red, green, and blue). By contrast, systems that allocate fewer bits of memory to storing color data can produce only images having a limited number of colors. For example, an 8-bit color image can include only 256 different colors.
On a color display device such as an LCD screen with a horizontal resolution of 800 pixels, the LCD screen can actually be composed of 800 red, 800 green, and 800 blue sub-pixels interleaved together (R-G-B-R-G-B-R-G-B . . . ) to form a linear array of 2400 single-color sub-pixels. Each sub-pixel is independently addressable, that is a color value can be set for each individual sub-pixel of the color display device. While each of the sub-pixels is individually addressable, the human eye sees (visible color gamut) a blending of the sub-pixels. For example, a single pixel wide white line can be produced by setting the input values of all sub-pixels for a row or column of pixels to a maximum value. The human eye does not ‘see’ closely spaced colors individually, and as such, cannot distinguish the individual color components. Instead, our vision system deliberately mixes the colors in combination to form intermediates, in this case the color white.
To display a fine structure monochrome image with fine detail such as black text on a white background or white text on a black background on a color display device or a monochrome display device, special attention must be paid to the visual output intensity of each sub-pixel in order to reduce color fringing effects. Unfortunately, device-specific pixel information that look good when used in displaying the text on one type of output display device may show color fringing effects when used in conjunction with other types of output display devices.
Color display devices may be constructed using different geometries of colored sub-pixels associated with each pixel. Depending on the color display device, different sub-pixel geometries result in various degrees of color fringing of monochrome images. Not all LCD screens, for example, have the same linear ordering of sub-pixels, for example a R-G-B ordering for a RGB color space type of output device. Other possible orderings include R-B-G, B-G-R, B-R-G, G-B-R and G-R-B. When two sets of images, one produced on an LCD device having a R-G-B sub-pixel geometry and the other produced on an LCD device having a B-G-R sub-pixel geometry such that neither LCD device displays color fringing, are displayed on a third LCD device having a R-G-B sub-pixel geometry, only the set of images with R-G-B ordering will appear without color-fringing. The set of images with B-G-R ordering will appear color-fringed. It would therefore be an advantage if the sub-pixel geometry for all pixel of an output device could be determined prior to display of an image so as to minimize the effect of color fringing.
For a given color display device, to minimize color fringing of finely detailed monochrome images, the proper intensity settings for each of the sub-pixels that make up a pixel as well as the sub-pixel geometry must be found.